Inductively linked receiver



Dec. 12, 1967 G. R. ABELL 3,357,434

INDUCTIVELY LINKED RECEIVER Filed April 6, 1964 2 Sheets-Sheet 1 GURDON R. ABELL INVENTOR.

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ATTORNEYS 3,357,434 Patented Dec. 12, 1967 3,357,434 INDUCTIVELY LINKED RECEHVER Gurdon R. Abel], West Woodstock, (301111., assiguor to Avco Corporation, (Iincinnati, Ohio, a corporation of Delaware Filed Apr. 6, 1964, Ser. No. 357,396 11 Claims. (Cl. 128-419) This invention relates generally to inductively-1inked devices for receiving electrical energy through the intact skin of a living body and more particularly to devices implanted in the human body for purposes of electrical stimulation, control, therapy, and the like. While the present invention will be described in connection with the stimulation of muscles, it is to be understood that it is not so limited and may be used Where it is desired to produce the flow of electrical current through a load such as live body tissue or conventional electrical devices or the like within a living body by transmitting through the intact skin electrical energy in the form of an alternating magnetic field.

he serious problem of complications involving the urinary tract in paraplegic patients, due to their inability to evacuate the bladder, has long been recognized, as has the advantage, in terms of avoidance of infection, of transmitting stimulating energy through the unbroken skin by means of radio-frequency induction. Accordingly, various studies have been made of the feasibility of electrically-stimulated evacuation through detailed experiments on animals and human patients.

For example, to provoke micturition, a stimulus of some volts into a somewhat capacitive tissue load of about 220 ohms is required with or even volts being required for some highly spastic cases. A typical stimulating signal used with the present invention comprises 4 millisecond pulses, 20 times per second, applied for 10 to 15 seconds every few hours of the waking day. All this energy amounts to about an order of magnitude greater than that required of a cardiac Pacemaker. 1

While an externally energized system would perhaps be the least complex, the ever-present possibility of infection overshadows the advantages of this system, and in a system wherein the energy is provided by a battery inside a patient, such a battery would have to be undesirably large to provide reasonable life. Regardless of which form of system is chosen, the receiver presents critical requirements: It must be sterile in all respects and nonirritating to surrounding tissue; it must be completely reliable even though immersed indefinitely in body fluids; the output leads must withstand endless flexing and all the requirements of surgical feasibility and circulation in the body in the vicinity of the receiver must be fully met.

In view of the importance of maintaining an unbroken skin on animals or persons in order to avoid infections, it is as previously noted generally agreed that a good way to bring electrical energy through the skin for long periods of time is by inductive rather than conductive means. However, useful transmission efiiciencies are attained by inductively-linked arrangements only if there is employed alternating magnetic fields having frequencies as high as possible without exceeding those where dissipation losses in subcutaneous tissue becomes an important contribution to overall losses and/ or injurious to the body. While the operating frequency of a receiver-transmitter system is not critical, the operating frequency should be as high as practical without exceeding the tissue loss limit of about 300 kilocycles. One must still keep in mind, however, that the indiscriminate application of such frequencies to the human body may be injurious and therefore should be limited to as small a region as possible.

An inductively-linked implantable receiver and its associated transmitter can be considered to be an inductively-linked two-tuned-circuit arrangement, one inductor of which is immersed in a dissipative medium such as, for example, subcutaneous tissues. Engineering analysis of such an arrangement points out factors which limit the efficiencies of conventional arrangements. First, the coupling between the sending and receiving circuits is considerably less than critical for the circuit Qs and spacingdiameter ratios generally employed. Also, for frequencies high enough to improve the Q of the usual receiving circuit, dissipative losses occur in the surrounding tissues, even in regions not directly between the sending and receiving inductors. This invention provides novel arrangements which yield good efficiency for practical values of inductor spacing-diameter ratio.

In addition to the above, useful efiiciencies are attained with conventional circuits only for large ratios of the diameter of the pickup coil to spacing between the pickup coil and the transmitter. The aforementioned large ratios are surgically impractical for such reasons as disturbance to the circulation in the tissues overlying the receiver coil. That this is so can be readily seen by considering that the circulation into such a region must come from its periphcry and that circulation into a region that is thin compared to its breadth could be seriously impeded. For this reason, conventional circuits must often be implanted at depths which are too great for satisfatcory electrical efficiency. As will now be seen, conventional circuitry cannot be used to provide a truly eificient and at once electrically and medically satisfactory implantable receiver.

It is also important to note that conventional circuits produce magnetic flux much of which never links the receiver pickup coil and which penetrates into regions deeper in the body than the pickup coil where it gives rise to dissipation, to the detriment of circuit efiiciency and the possible detriment of underlying tissues or organs.

Further, as a result of experiments with the present invention, it is believed that electrical energy used at least for stimulation should be completely biphasic, that is, such electrical energy should be without any long-term average direct current component since it is believed that the presence of a direct current component causes so-called tissue corrosion which may result in necrosis of the organ stimulated and/or the complete rejection by the body of at least the electrodes of an implanted receiver.

A still further requirement of an inductively-coupled receiving device implanted in living tissue is that it be sensitive only to energy projected to it from a given direction over a short distance to prevent accidental actuation of the receiver.

In accordance with the present invention, there is provided a resonant circuit comprising a pickup coil and a capacitor. A ferromagnetic member is in abutting rela tionship with one end of the coil. The coil is preferably wound on a plastic spool which includes the ferromagnetic member. Included in the center of the coil along with the aforementioned capacitor is such additional circuitry as may be needed to achieve the results desired therefrom. In a preferred embodiment for stimulating the detrusor muscle of the bladder of a human patient this circuitry comprises a rectifier for providing pulses of electrical energy, when pulses of radio-frequency current are induced in the pickup coil, and a filter circuit for removing the direct current component of these pulses. Output leads couple the output signal of the filter circuit to the detrusor muscle. The above-described structure is immersed in an impermeable, monolithic block of thermosetting material. This is preferably postcured to render it nonirritating to body tissue and then is coated with a jacket of nonirritating medical-grade silicone synthetic rubber, such as, for example, Medical Silastic 382, which is also postcured. Medical Silastic 382 is obtainable from Dow-Corning Corp., Midland, Michigan.

The present invention meets all of the previously-noted requirements of implantable, inductively-linked receives. The ferromagnetic member permits, among other things, the lowest obtainable diameter to spacing ratio while still providing good electrical efiiciency, it reduces to a negligible value the penetration of magnetic flux into the body cavity, it minimizes the magnetic flux in the skin between the receiver and transmitter, and it improves the coefficient of coupling with the transmitter by shaping the flux field. It. also provides the optimum in size and officiency. The ferromagnetic member, by providing an effectively-short magnetic path which lies behind the pickup coil as viewed from the transmitter, causes the magnetic flux from the transmitter to preferentially link the pickup coil rather than avoid it and also shunts out flux which would otherwise penetrate into regions behind the pickup coil. By causing the flux preferentially to link the pickup coil, the ferromagnetic member increases the effective coefficient of coupling between the transmitter and receiver pickup coils for any given ratio of receiver pickup coil diameter to spacing between the transmitter and receiver pickup coils. On the other hand, for magnetic flux projected toward the pickup coil from the side having the ferromagnetic member, that member will shunt the flux before it reaches the pickup coil thus rendering the coil insensitive to magnetic flux from this undesired direction.

The novel features that are considered characteristic of the invention are set forth in the appended claims; the invention itself, however, both as to its organization and method of operation, together with additional objects and advantages thereof, will best be understood from the fol lowing description of a specific embodiment when read in conjunction with the accompanying drawings, in which:

FIGURE 1 is a schematic wiring diagram of a receiver in accordance with the present invention;

FIGURE 2 is a side view partly in section of a receiver in accordance with the present invention; and

FIGURE 3 is a sectional top view taken on lines 33 on FIGURE 2.

Directing attention now to FIGURE 1, there is shown a pickup coil and a capacitor 11 which comprises a resonant circuit 12. The input terminal 13 of a rectifier 14 is connected as at 15 intermediate the ends of coil 10. A second capacitor 16 and a resistor 17 are connected in parallel and between the output terminal 18 of the rectifier 14 and one end 19 of the pickup coil 10. A third capacitor 21 is connected between the output terminal 18 of the rectifier 14 and the output lead 22. A second output lead 23 is connected to the end 19 of the pickup coil 10.

The resonant circuit 12 is tuned to a predetermined frequency, such as, for example, 94.5 kc. and the rectifier 14 in combination with capacitor 16 and resistor 17 functions to provide a rectified output signal from the resonant circuit 12, i.e., pulses of electric energy when pulses of 94.5 kc. radio-frequency current are inductively induced in the resonant circuit 12. Resistor 17 and capacitor 21 perform the function of separating the direct current component from the alternating component in the aforementioned pulses of electric energy, i.e., they provide an output signal, on leads 22 and 23, without any average direct current component. The output leads 22 and 23, the majority of which are covered with a medically-suitable insulating material, couple the output signal of the receiver to the load, such as, for example, the detrusor muscle of the bladder.

Output leads which have operated satisfactorily were small wires comprised of 49 strands of stainless steel covered with a braid of synthetic fibre of a type tolerable by living tissue, which was in turn coated with medicalgrade silicone syntheticrubber, again of a type tolerable by living tissue. Where the bare ends of the output leads are to be stitched into a tissue to provide electrical contact therewith, these ends, as best shown in FIGURE 2, are preferably provided with a crimped sleeve 25 to prevent unraveling, and the ends of the output leads are crimped in the open end of a hollow curved stitching needle 26. Thus, after each lead has been stitched into a tissue, the lead may be severed between the sleeve and the needle, and the needle with its short portion of the output lead thrown away.

In a receiver that operated successfully, the resonant circuit comprised a cylindrical pickup coil comprising 240 turns of 15/44 Litz wire and a .0022 mfd. 600 volt ceramic capacitor. The rectifier 14 was comprised of two small silicon rectifiers connected in parallel (for reasons of safety); the resistor 17 was a 3,000 ohm, watt resistor; capacitor 16 was a 1.0 mfd., 20 volt DC tantalum capacitor; and capacitor 21 was comprised of two 2.7 mfd., 15 volt DC tantalum capacitors connected in parallel. The parallel connection of the aforementioned two 2.7 mfd. capacitors was used to conserve space. The rectifiers 14, resistors 17-, and capacitor 16 were connected across about 35 turns of the pickup coil and the pickup coil was tuned to a bogie frequency (a frequency which is known to become shifted to the design frequency after and as a result of encapsulation) prior to encapsulation of the entire receiver in a thermosetting material which at the same time protects the receiver components from attack by the corrosive tissue fluids and prevents damage to tissues from irritating materials in the receiver components or in the encapsulating material itself. The use of the smallest available reliable circuit components, constructed and combined in accordance with the present in vention as more fully described hereinafter, permits the fabrication of a receiver about 1% inches in diameter, V inch thick, and weighing less than one ounce.

Consider now for a moment prior art or conventional inductively coupled circuits which includes a receiver which supplies direct current or pulses to living tissue or electrical equipment having a load resistance of the order of -1000 ohm-s. In such arrangements the transmitter and recever are m-ulti-turn coils aligned along their axis of symmetry, and the living tissue or electrical equipment which comprises the useful load is generally connected, either directly or through a rectifier, directly across the coil of the receiver. Even when the receiving coil is tuned, by a capacitor across it, to resonance with the frequency of the transmitter, and even when an extraordinarily low inductance-to-capacitance ratio is employed, the loaded Q of the conventional receiver is still too low to provide reasonably efiicient transfer of energy at the coefficient of inductive coupling resulting from the ratio, of receiver coil diameter to spacing between coils, which is surgically feasible. The upper limit on this ratio is set by such factors as the maximum diameter of the receiving coil which can be tolerated, the minimum depth below the body surface at which it can be surgically implanted, and the maximum ratio of both factors at which circulation is adequate in the tissues overlying the receiving coil. Further, in conventional arrangements, much of the flux never links the pickup coil and much of the flux penetrates into regions deeper in the body where it gives rise to dissipation, to the detriment of circuit efiiciency and possibly, to the detriment of underlying tissues or organs.

The problem of coupling the penetration of flux can of course be reduced if the transmitter coil and pickup coil are provided with conventional cup cores of high resistivity ferromagnetic material. However, the use of a cup core drastically increases the size of the pickup coil because it surrounds all but one end of the pickup coil and, accordingly, extends out past the pickup coil and also con sumes all of the available space in the coil, thereby sacrificing much if not all of the advantages gained by its use.

Consider now FIGURE 2 and FIGURE 3 which showv constructional details of the present invention. As shown in these figures, the coil 10 is wound on a plastic spool designated generally by the number 30. The spool 30, composed of a suitable thermosetting plastic, such as, for example, a copolymer of epoxy and polyamide in such ratio as substantially to combine completely with each other, includes an outwardly extending lip 32 and a cylindrical sleeve portion 33 having a stepped rear portion 34. An annular high resistivity ferromagnetic member 31 is carried by the rear portion 34 and when mounted in place forms the second lip of the spool 30. The rear portion 34 is stepped inwardly to receive a support member or circuit board 35 in its open center portion at shoulder 36 and the ferromagnetic member 31 at its shoulder 37. The previously-described circuit components are mounted on the circuit board 35 in conventional manner and interconnected on the side thereof opposite from the components. A passage 38 is provided in the circuit board to facilitate encapsulation. The ferromagnetic member 31 is preferably first bonded in place and the coil thereafter wound on the spool so that the fiat end of the coil is in abutting engagement with the ferromagnetic member 31.

When all of the components, including the output leads, have been assembled and the coil tuned to a suitable bogie frequency, the entire combination as previously noted is encapsulated in an impermeable and preferably biologically-inert material 39 such as the epoxy-polyamide copolymer mentioned previously. Encapsulation may be accomplished by use of conventional and well known vacuum techniques which insure complete impregnation. Lip 32 preferably extends outwardly past the outer periphery of coil 10 and ferromagnetic member 31 to assure centering of the entire combination in the encapsulation mold, to the end that a sufiicient thickness of encapsulating material be provided over the entire combination. The encapsulated receiver is now preferably postcured to render it substantially nonirritating to body tissue and is then coated with a jacket of nonirritating and biologically-inert material 40, such as for example medical-grade silicone rubber, which is then also postcured.

Particular attention is drawn to the action of ferromagnetic member 31 employed in the arrangement shown in FIGURE 2 and FIGURE 3. By providing a short and easy magnetic path from the inside to the outside of coil 10, on the side farthest fro-m the transmitting 0011, it causes transmitted flux preferentially to link coil 10, thus providing a marked increase in the effective coefiicient of coupling between transmitter and receiver coils, and a concomitant increase in transfer efiiciency for a given ratio of receiver coil diameter to spacing between the transmitter and receiver coils. Indeed, the small thin ferromagnetic member 31 yields almost as good coupling and prevention of undesired field penetration as does a full cup core, especially at surgically useful ratios of receiver coil diameter to spacing between transmitter and receiver coils. In addition, the arrangement shown in FIGURE 2 and FIGURE 3 facilitates winding the coil, permits maximum utilization of the space within the coil and the ferromagnetic member and provides the smallest possible outside diameter.

For completeness, the receiver has been shown covered by a jacket of biologically-inert material as required for implantation in a living animal or person. Minor modlfications of the ferrite member may be made without departing from the scope of the present invention. For example, the hole of the ferrite member may be made larger at, however, the expense of reduction in performance, or made smaller, or even omitted, with a minor beneficial effect on performance. It has been found, however, that the inner periphery of the ferrite member can be as large as the inner periphery of the coil without appreciable loss of performance, thus providing more space in the open center portion of the pickup coil for the circuit components. Also, the outer periphery of the ferrite member may be slightly larger or smaller than that of the pickup coil, again, however, at the expense of respectively an increase in size or a decrease in performance. For example, a thin outside cuff on the ferrite member, somewhat like a rudimentary outside cylindrical portion of a cup core, has a slight beneficial effect with only a minor increase in the total size of the pickup coil. However, because an outside cuff on the ferrite member makes the manufacture and assembly of the receiver appreciably more difficult without a concomitant beneficial effect, its

- use is not recommended. One may, however, round or chamfer the outside edge of the ferrite member in order to simplify the task of making a thin leak-proof case which encloses the entire assembly.

Returning now to FIGURE 1, attention is directed to the connection of rectifier 14 to point 15 intermediate the ends of coil 10. It has been found that the unloaded impedance of the ferromagnetically-loaded and shielded tuned circuit comprising coil 10, ferromagnetic member 31, and capacitor 11 is greater than that of a load represented by living tissue. The connection of rectifier 14 intermediate the ends of coil 10 functions toincrease the loaded Q of the receiver by transforming the impedance of its load when implanted in a human body to a higher value more nearly commensurate with the unloaded impedance of the aforementioned ferromagnetically-loaded and shielded tuned circuit. This transformation is achieved by connecting the input terminal of the rectifier to a point intermediate the ends of coil 10. For a load of, for example, 220 ohms with an operating frequency of 94.5 kilocycles per second, a tuning capacitor of 0.0022 microfarad, and an unloaded Q of for the combination of pickup coil 10 and ferrite member 31, if one desires a ratio of unloaded Q to loaded Q of about 6, then the number of turns in the pickup coil between points 15 and 19 should be about one-seventh of the total number of turns in the pickup coil. If a higher loaded Q is required, as would be the case when the surgically-attainable ratio of pickup coil spacing to pickup coil diameter is greater than usual, then a smaller fraction of the turns in the pickup coil 10 may be included between points 15 and 19, with, however, a concomitant decrease in the etficiency of the receiver. Because of the presence of the ferromagnetic member, it is important that the receiver be orientated such that the pickup coil is between the ferromagnetic member and the surface of the skin. To obtain good efficiency, the receiver should be implanted subcutaneously with its outer surface less than about three-fourths of an inch from the surface of the skin.

The foregoing description of a specific embodiment of the invention is for the purpose of illustration rather than limitation. Obviously, components may be omitted, other components may be used, and the design can be modified by those skilled in the art to suit the particular application to be served without departing from the novel aspects of the invention as defined by the following claims.

I claim:

1. In an inductively-linked device for receiving electrical energy within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a generally flat end portion and an open center portion;

(b) a substantially-flat ferromagnetic member having an outer periphery substantially the same as that of said coil, said member being in substantially abutting relationship with said fiat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(0) a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) first means disposed in the open center portion of said coil for coupling electrical energy out of said resonant circuit;

(e) second means for coupling the output of said first means to a load and (f) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said resonant circuit, ferromagnetic member, first means, and a portion of said second means.

2. In an inductively-linked device for receiving electrical energy Within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a generally fiat end portion and an open center portion;

(b) a substantially-flat ferromagnetic member having an outer periphery substantially the same as that of said coil, said member being in substantially abutting relationship with said fiat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(c) a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

((1) first means disposed in the open center portion of said coil for coupling out of said resonant circuit electrical energy having a substantially-zero direct current component;

(e) second means for coupling the output of said first means to a load; and

(f) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said resonant circuit, ferromagnetic member, first means, and a portion of said second means.

3. In an inductively-linked device for receiving electrical energy within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a generally fiat end portion and an open center portion;

(b) a substantially-fiat ferromagnetic member having an outer periphery substantially the same as that of said coil, said member being in substantially abutting relationship with said flat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

() a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) means disposed in the open center portion of said coil for coupling electrical energy out of said resonant circuit;

(e) a .pair of flexible output leads for coupling the output of said means to a load; and

(f) e1ectrically-nonconductive, impermeable material nonirritating to live body tissue encasing said resonant circuit, ferromagnetic member, first means, and a portion of said output leads.

4. In an inductively-linked device for receiving electrical energy within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a predetermined number of turns, a generally flat end portion, and an open center portion;

(b) an annular and substantially-fiat ferromagnetic member having an inner and outer periphery substantially the same as that of said coil, said member being in substantially abutting relationship with said fiat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(c) a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) means disposed in the open center portion of said coil and coupled across less than all of said turns of said coil for coupling electrical energy out of said resonant circuit;

(e) a pair of flexible output leads for coupling the output of said means to a load; and

(f) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said resonant circuit, ferromagnetic member, first means, and a portion of said output leads. 5. In an inductively-linked device for receiving radio frequency energy Within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a generally fiat end portion and an open center portion;

(b) an annular and substantially-flat ferromagnetic member having an inner and outer periphery not substantially greater than that of said coil, said member being in substantially abutting relationship with said fiat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(0) a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) a rectifier disposed in the center of and coupled to said coil for providing a rectified output signal from said resonant circuit;

(e) filter means coupled to the output of said rectifier for providing an output signal;

(f) output leads coupled to the output of said filter means and adapted to be attached to a load for coupling said output signal of said filter means to said load; and

(g) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said coil, ferromagnetic member, capacitor, rectifier, filter means, and a portion of said output leads.

6. In an inductively-linked device for receiving radio frequency energy Within a living body and producing a fiow of electrical current through live body tissue, the combination comprising:

(a) an annular pickup coil having a generally fiat end portion and an open center portion;

(b) an annular and substantially-fiat ferromagnetic member having an inner and outer periphery not substantially greater than that of said coil, said member being in substantially abutting relationship with said flat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(0) a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) a rectifier disposed in the center of and coupled to said coil for providing pulses of electrical energy from said resonant circuit;

(e) filter means coupled to the output of said rectifier for removing the direct current component in said pulses of electrical energy;

(f) output leads coupled to the output of said filter means and adapted to be attached to live body tissue for coupling said pulses of electrical energy from said filter means to live body tissue; and

(g) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said coil, ferromagnetic member, capacitor, rectifier, filter means, and a portion of said output leads.

7. In an inductively-linked device for receiving radio frequency energy Within a living body and producing a flow of electrical current through live body tissue, the combination comprising:

(a) an annular pickup coil having a generally flat end portion and an open center portion;

(b) an annular and substantially-flat ferromagnetic member having an inner and outer periphery not substantially greater than that of said coil, said member being in substantially abutting relationship with said flat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(0) a capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) a rectifier disposed in the center of and coupled to said coil intermediate its ends for providing pulses 9 of electrical energy from said resonant circuit;

(e) filter means coupled to the output of said rectifier for removing the direct current component in said pulses of electrical energy;

(f) output leads coupled to the output of said filter means and adapted to be attached to live body tissue for coupling said pulses of electrical energy from said filter means to live body tissue; and

(g) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said coil, ferromagnetic member, capacitor, rectifier, filter means, and a portion of said output leads.

8. In an inductively-linked device for receiving radio frequency energy Within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a generally flat end portion and an open center portion;

(b) a substantially-fiat ferromagnetic member having an outer periphery substantially the same as that of said coil, said member being in substantially abutting relationship with said flat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(c) a first capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) a rectifier having a first input terminal and a first output terminal disposed in the center portion of said coil, said first input terminal being connected to said coil;

(e) a second capacitor and a resistor disposed in the center portion of said coil connected in parallel and between said first output terminal and one end of said coil whereby said rectifier, second capacitor, and resistor are connected across a predetermined number of turns of said coil;

(f) a third capacitor disposed in the center portion of said coil having a second input terminal and a second output terminal, said second input terminal being connected to said first output terminal;

(g) a pair of flexible output leads adapted to be coupled to a load connected respectively to said second output terminal and said one end of said coil; and

(h) electrically-nonconductive, impermeable material nonirritating to live body tissue encasing said coil, ferromagnetic member, capacitors, resistor, and a portion of said output leads.

9. In an inductively-linked device for receiving radio frequency energy within a living body and producing a flow of electrical current through a load, the combination comprising:

(a) an annular pickup coil having a generally flat end portion and an open center portion;

(b) an annular substantially-flat ferromagnetic member having an inner and outer periphery substantially the same as that of said coil, said member being in substantially abutting relationship with said flat end portion of said coil and having a thickness dimension that is small compared to that of said coil;

(c) a first capacitor disposed in the center of and connected across said coil to form a resonant circuit;

(d) a rectifier having a first input terminal and a first output terminal disposed in the center portion of said coil, said first input terminal being connected to said coil intermediate its ends;

(e) a second capacitor and a resistor disposed in the center portion of said coil connected in parallel and between said first output terminal and one end of said coil whereby said rectifier, second capacitor and resistor are connected across less than all of the turns of said coil;

(f) a third capacitor disposed in the center portion of said coil having a second input terminal and a second output terminal, said second input terminal being connected to said first out terminal;

(g) a pair of flexible output leads adapted to be coupled to a load connected respectively to said second output terminal and said one end of said coil; and

(h) e1ectrically-nonconductive, impermeable material nonirritating to live body tissue encasing said coil, ferromagnetic member, capacitors, resistor, and a portion of said output leads.

10. The combination as defined in claim 9 wherein said coil is wound on an electrically-nonconductive spool comprising a thin-walled annular sleeve portion and a thin, outwardly-extending lip at one end of said sleeve portion, said lip extending outwardly to about the outer periphery of said coil, said ferromagnetic member being carried by said sleeve portion remote from said lip, and an electrically-noneonductive support member disposed within said sleeve for supporting said rectifier, capacitors, and resistor.

11. The combination as defined in claim 10 wherein the outer and inner surface of said sleeve are each provided with a shoulder for abutting engagement with respectively said ferromagnetic member and said support member.

References Cited FOREIGN PATENTS 9/ 1943 Great Britain.

RICHARD A. GAUDET, Primary Examiner. W. E. KAMM, Assistant Examiner. 

1. IN AN INDUCTIVELY-LINKED DEVICE FOR RECEIVING ELECTRICAL ENERGY WITHIN A LIVING BODY AND PRODUCING A FLOW OF ELECTRICAL CURRENT THROUGH A LOAD, THE COMBINATION COMPRISING: (A) AN ANNULAR PICKUP COIL HAVING A GENERALLY FLAT END PORTION AND AN OPEN CENTER PORTION; (B) A SUBSTANTIALLY-FLAT FERROMAGNETIC MEMBER HAVING AN OUTER PERIPHERY SUBSTANTIALLY THE SAME AS THAT OF SAID COIL, SAID MEMBER BEING IN SUBSTANTIALLY ABUTTING RELATIONSHIP WITH SAID FLAT END PORTION OF SAID COIL AND HAVING A THICKNESS DIMENSION THAT IS SMALL COMPARED TO THAT OF SAID COIL; 